Recently, transparent materials capable of efficient frequency upconversion, most being various rare-earth ion-doped fluoride glasses and crystals, have received great attention due to the possibilities of utilizing these materials to achieve blue or green solid state lasers. While no significant difference in upconversion efficiency is observed between fluoride glasses and single crystals, single mode optical fiber doped with a low level of rare-earth ions can be drawn from fluoride glasses, bringing about highly efficient blue or green upconversion fiber lasers. Unfortunately, heavy metal fluoride glasses suffer certain undesirable attributes which have restricted their applications. Most notably, heavy metal fluoride glasses exhibit poor resistance to devitrification. U.S. Pat. No. 4,674,835 to Mimura et al. discusses the crystallization problems of heavy metal fluoride glasses, one example of which is called ZBLAN, and the light scattering problems resulting therefrom.
The great susceptibility of heavy metal fluoride glasses to devitrification also generates problems in forming large preforms. Crystallization at the interface between the core and cladding, during the production of the preform, causes problems in the most commonly used methods for preparing an optical fiber. That is, heavy metal fluoride glasses are quite prone to inhomogeneous nucleation, the consequence of which being crystallization at the core and cladding interfaces, particularly during the drawing of the optical fiber. The resulting fibers are subject to serious scattering losses due to crystals in the fibers.
Devitrification of the heavy metal fluoride glasses is aggravated when ions necessary to impart differences in indices of refraction to the core and cladding are added to the glass composition. Additional doping, for example, with rare-earth metal ions, also tends to reduce the stability of the glass. As a consequence of those problems, research has focused on finding additives which will reduce the tendency of the glass to devitrify and to increase the chemical stability thereof. In addition, the preparation of fluoride glasses requires the glass forming components to be reheated to their softening temperatures, which generally are about 75.degree. C. above the glass transition temperatures. In addition, fluoride glasses cannot be melted in air but require a water-free, inert gas environment.
Most oxide glasses (such as silica dioxide) are much more chemically and mechanically stable, are easier to prepare, and are more easily fabricated into rods, optical fibers, or planar waveguides than fluoride glasses. Unfortunately, due to their larger phonon energy, silica glasses are very inefficient for infrared upconversion. It has also been shown that addition of oxides into some fluoride glasses improve their stability, but this is not preferred, since even a small addition of oxides will significantly quench the upconversion luminescence. Early in 1975, Auzel et al., J. Electrochem. Soc., 122:101 (1975) reported an interesting class of infrared ("IR" ) upconversion materials which were prepared from classical glass forming oxides (SiO.sub.2, GeO.sub.2, P.sub.2 O.sub.6, etc. with PbF.sub.2 and rare-earth oxides), and showed an efficiency nearly twice as high as LaF.sub.3 :Yb:Er phosphor. Since these kinds of materials were comprised of a mixture of glassy and crystalline phases, and the embedded crystals were very large in size (around 10 .mu.m), they were not transparent.
In Wang et al., "New Transparent Vitroceramics Codoped With Er.sup.3+ and Yb.sup.3+ For Efficient Frequency Upconversion," Appl Phys. Lett, 63(24):3268-70 (1993), transparent oxyfluoride vitroceramics (also called glass-ceramics) containing oxides of large phonon energy, like SiO.sub.2 and AlO.sub.1.5, but showing IR to visible upconversion more efficient than fluoride glass was described. The composition of Wang contained, expressed in terms of mole percent,
SiO.sub.2 30 CdF.sub.2 20 AlO.sub.1.5 15 YbF.sub.3 10 PbF.sub.2 24 ErF.sub.3 1
The glass produced from that composition was heat treated at 470.degree. C. to develop nanoocrystallites which the authors stated did not reduce the transparency of the body. The authors posited that the Yb.sup.3+ and Er.sup.3+ ions were preferentially segregated from the precursor glass and dissolved into the nanocrystals upon heat treatment. The size of the nanocrystallites was estimated by the authors to be about 20 nm; that size being so small that light scattering loss was minimal. The authors reported the upconversion efficiency of their products to be about 2 to 10 times as high as that measured on the precursor glass and other fluoride-containing glasses. However, the crystals which are formed in the Wang glass have a cubic lattice structure and this limits the concentration of some of the trivalent rare-earth elements which may be incorporated into the crystal phase. Another problem with these materials is that they require cadmium in the formulation. Cadmium is a carcinogen and, thus, its use is restricted. Hence this type of glass would not be desirable for any large scale manufacturing operation.
The present invention is directed toward overcoming these above-noted deficiencies,